Polyurethanes and polyurethane-ureas are widely used in the manufacture of various products such as expanded foam for padding, elastomers for the automotive industry, and have a density and rigidity which varies over a wide range. These properties depend on numerous factors including the structure of the reactants, and on the used additives and catalysts.
The improvement of a specific property inevitably entails the modification of other properties since it is obtained by modifying a component or the preparation process, hence obtaining the combination of desired properties is very difficult or even impossible.
This limit is particularly true when the material must combine good mechanical properties such as resistance to loads and vibrations, which require a relatively rigid structure, with a high elastic recovery of the form after compressive stress is released (low compression set) which is generally obtained with softer polymers.
When, in addition to the above-mentioned combination, good resistance to abrasion and adequate chemical resistance are required, the possibility of finding a material which has all the desired properties is further reduced.
This is typically the case of materials used in variable and/or temporary load or vibration conditions, in the presence of moving mechanical parts, in various temperature and humidity conditions, in which the elastic deformation must be immediately recovered.
An example of said materials is represented by the shock absorbers for railway lines positioned below the sleepers and/or below the rails (railway bumpers and railway pads) as such and/or through supports and/or installation structures: they must have, in addition to a low compression set, generally below 20, a high resistance to static and dynamic loads (vibrations), and therefore a static rigidity (ratio between load and deformation sustained) and dynamic rigidity as required by the specifications of the sector.